This invention relates generally to gas turbine engines and, more particularly, to combustion systems relating thereto.
The invention herein described was made in the course of or under a contract, or a subcontract thereunder, with the United States Department of the Air Force.
Recent efforts to reduce emissions in gas turbine engines have brought about the use of staged combustion techniques wherein one burner or set of burners is used for low speed, low temperature conditions such as idle, and another, or additional, burner or burners are used for higher temperature operating conditions. One particular configuration of such a concept is that of the double annular combustor wherein the two stages are located concentrically in a single combustor liner. Conventionally, because of the necessity for having an igniter in the pilot stage, and the relative ease with which it can be installed in the outer liner, the pilot stage section is located concentrically outside and operates under low temperature and low fuel/air ratio conditions during engine idle operation. The main stage section, which is located concentrically inside, is later fueled and cross-ignited from the pilot stage to operate at the high temperature and relatively high fuel/air ratio conditions.
It will be recognized by one skilled in the art that in a double annular combustor, where the height of the dome is always greater than that of the turbine nozzle annulus, the outer section is going to be generally straight and of short length, and the inner section is going to be generally curved and have a longer length. It has been recognized that these structural relationships are disadvantageous for a number of reasons. In regard to emissions, when the engine is operating at idle, the pilot stage is operating with low compressor discharge temperatures and pressures and the reaction rates are thus relatively slow. Accordingly, in order to allow for complete or near-complete combustion to thereby reduce the amount of hydrocarbon and carbon monoxide emissions, it is preferable to have a long residence time, a characteristic which is not inherent in the shorter radially outside pilot section. On the other hand, in the main stage section, where nitrous oxides are the primary emission problem, it is desirable to minimize the residence time so as to also minimize the forming of nitrous oxides. Again, locating of the main stage section on the radially inner side tends to defeat this purpose since this section is necessarily longer than that on the outer side.
Another disadvantage of having the main burner on the radially inner side is that the higher temperature gases emanating from that burner tend to flow against the curved inner liner of the combustor. Thus, it is necessary to provide a high degree of cooling to that liner in order to prevent it from burning through.
A further condition which renders the conventional double annular combustor configuration inadequate is that of the resulting natural profile at the turbine nozzle annulus. Ideally, in order to optimize turbine life, it is desirable to have a profile in which the temperatures are cooler at the inner diameter than at the outer diameter. However, with the main burner located radially inward as described hereinabove, the profile which exists is one having hotter temperatures toward the turbine inner side.
It is therefore an object of the present invention to provide a double annular combustor with reduced emission operating characteristics.
Another object of the present invention is the provision of a double annular combustor with improved structural integrity.
Yet another object of the present invention is the provision in a double annular combustor for minimizing the impingement of hot gases against the liner of the combustor.
Still another object of the present invention is the provision in a double annular combustor for a turbine inlet temperature profile which is cooler on the radially inner side.
Yet another object of the present invention is the provision of a double annular combustor which is economical to manufacture and efficient and effective in use.
These objects and other features and advantages become more readily apparent upon reference to the following description when taken in conjunction with the appended drawings.